What is meant by protein quality?

Protein quality, from a nutrition perspective, is a term used to describe how well a protein from food matches the body's requirements and, therefore, how useful the protein is for our body. This is determined by looking at the building blocks which make up the protein; called Amino Acids.

There are 20 primary amino acids, 9 of which our body cannot make on its own. These are considered "essential" and so must come from our diet. A protein's quality is determined by asking the following questions:

# Are all the 9 essential amino acids present in the protein?

# Are the ratios of essential amino acids in the protein ideal?
The first question is answered fairly easily with a "yes". In fact, pretty much every source of protein has at least some of all the essential amino acids. However, if the protein is deficient in 1 or more essential amino acid then it is considered incomplete. For your reference, grains are typically low in the amino acid lysine, while legumes are low in methionine. Animal products, on the other hand, are high in all the essential amino acids and are usually considered complete. (the exception being gelatin)

Answering the second question is a bit more complex. Basically, the body needs a certain proportion of essential amino acids in the protein in order for it to be used most effectively. If all 9 are present yet 1 or more are not very abundant then the protein is of lower quality because the body will not be able to use the protein to its fullest potential.

The "gold standard" used when determining protein quality is the egg (typically chicken egg). Eggs have all 9 essential amino acids in its protein and the ratios of these amino acids are very similar to the ideal protein needed by the body. You simply can't find better protein in your diet than egg protein! As a result, egg protein is considered the #1 quality protein and all other proteins from all other food sources are compared against this standard.


http://www.faqs.org/nutrition/Pre-Sma/Protein.html
http://en.wikipedia.org/wiki/Essential_amino_acids